A human spinal column includes vertebral bodies alternating with intravertebral discs extending from the neck to the pelvis. The discs generally form strong joints, separate, cushion and allow flexure and torsion between the vertebrae.
When functioning properly, the vertebrae and discs allow a person to bend forward, backward, sideways and to twist. To accomplish this, the discs typically permit adjacent vertebrae six degrees of motion: vertical (compressing to absorb shock and tension), bending forward and backward, bending to the sides and twisting. The cervical and lumbar discs also can be thicker anteriorly to contribute to lordosis. Thoracic discs usually are more uniform. Unfortunately, disc disease may limit spinal motion or cushioning or permit the motion with pain.
Each intervertebral disc usually has a central area composed of a colloidal gel, called the nucleus pulposus, on a surrounding collagen-fiber composite structure, the annulus fibrosus. The nucleus pulposus typically occupies 25-40% of the disc's total cross-sectional area. The nucleus pulposus usually contains 70-90% water by weight and may mechanically function like an incompressible hydrostatic material. The annulus fibrosis surrounds the nucleus pulposus and typically resists torsional and bending forces applied to the disc. The annulus fibrosis thus often serves as the disc's main stabilizing structure. The annulus fibrosus usually resists hoop stresses due to compressive loads and the bending and torsional stresses produced by a person bending and twisting. The fibers of the annulus form lamellae, individual layers of parallel collagen fibers, that attach to the superior and inferior end plates of adjacent vertebrae. Vertebral end-plates separate the disc from the vertebral bodies on either side of the disc.
The anterior longitudinal ligament, which is anterior to the vertebral bodies, and the posterior longitudinal ligament, which is posterior to the vertebral bodies and anterior to the spinal cord function to hold the spinal structure together. The muscles of the trunk provide additional support.
Trauma or disease may displace or damage spinal discs. A disc herniation occurs when annulus fibers weaken, and the inner tissue of the nucleus (nucleus pulposus) bulges out of the annulus. The herniated nucleus may compress a spinal nerve, which could result in pain, lack of sensation, loss of muscle control or even paralysis. Alternatively, disc degeneration may result when the nucleus deflates. Subsequently, the height of the nucleus decreases often causing the annulus to buckle in areas where the laminated plies are loosely bonded. This also may cause chronic and severe back pain. Further, the disc may rupture, resulting in a portion of the nucleus pulposus flowing through the fractured annulus, outside the disc to compress nerves and/or the spinal cord. This material may irritate the spinal nerve or spinal cord when tit flows into a posterior region of the disc.
Whenever the nuclear tissue is herniated or the disc degenerates, the vertical disc space typically narrows and the adjacent vertebra may lose much of their normal stability. In many cases, to alleviate pain from degenerated or herniated discs, a surgeon removes the nucleus by performing a discectomy, which requires surgical dissection. The healing from such a surgery usually causes scar tissue which may compress the same or nearby nerves and/or spinal cord (which were being affected by the now-removed nucleus) and cause chronic pain and nervous system dysfunction. Other times, if the disc is compressed, the surgeon may perform a surgical fusion of two or multiple adjacent vertebrae together. While this treatment may or may not alleviate the pain and nervous dysfunction, the patient often loses all disc motion in the fused segment. Ultimately, this procedure places greater stress on the discs adjacent to the fused segment as the adjacent discs compensate for lack of motion.
In the case of severe disc degeneration, the height of the disc often is flattened to such an extent that the adjacent vertebral body bones touch and eventually grow together. This may stop pain by stopping the movement of the disc between the vertebral bones, and is known as an auto-fusion.